1. Field of the Invention
The present invention relates to an image reading device for reading an image on an original sheet, and an image forming device for forming an image based on digital image data read by the reading device, more specifically, a digital copying machine which blanks the background other than the original sheet by electrically judging the size of the original sheet from the data read out.
2. Description of the Related Art
In conventional copying machines, a white reflection plate on the original sheet cover avoids blackening of the section other than the original sheet when an original sheet having a size smaller than the set sheet size is copied. When copying an original sheet, the sheet is placed on the original sheet glass, and the front surface of the sheet glass is covered by the white reflection plate. However, with the conventional technique, when a three-dimensional object is copied, the reflection plate cannot be set properly on the original, rendering the background section of the original on a copying sheet gray or black.
Some conventional copying machines include a frame erasing function for the purpose of overcoming such a drawback. The frame erasing function detects the size of a sheet used for copy, and controls copying so as not to form any image in the sheet's periphery section of a certain width. The conventional frame erasing function has a member for erasing the unwanted section in advance by exposing the section to light from its light source before developing the static latent image.
In other conventional copying machines, the position and the size of an original is detected before start of a copying machine, and only the detected region of the original is copied. More specifically, there is a known method in which a metal reflection plate is used for the original sheet press surface of the sheet cover, and pre-scanning is performed with a light beam so as to detect the four corners of the original based on the difference between the intensity of the reflection light from the original and that from the original press surface. There is also a known method in which the original press surface of the original cover is coated with a fluorescent paint, and pre-scanning is performed by use of a filter transmitting only the light having a certain wavelength so as to detect the four corners of the original based on the difference between the intensity of the reflection light from the original and that from the original press surface.
The frame erasing function using a light source is effective in the case where an original image is copied on a copy sheet at substantially the same magnification; however in the case where the original image is small, the periphery of the original sheet is inevitably blackened. In the method of detecting the position and size of an original before start of a copying operation, detection of the size is in many cases based on typical sizes such as A4 and B4. Further, when the original is placed inappropriately, it is judged to be a wrong size.
As described above, with the conventional copying machines, the automatic sheet size detection function cannot detect any sheets not rectangular. Further, with copying machines having an original press coated with fluorescent paint, the sheet size cannot be detected when the original press is not appropriately set on the original sheet.